Process for lowering nitrogen oxides in effluent gases

ABSTRACT

A process for reducing the level of nitrogen oxide contaminants in an industrial effluent gas comprises (a) oxidizing said gas to convert the contaminants substantially to a mixture of N 2  O 3  and NO 2  and (b) passing the oxidized gas through an aqueous medium containing an amount of hydrogen peroxide maintained at a level just sufficient to oxidize the N 2  O 3  and NO 2  to HNO 3 .

BACKGROUND OF THE INVENTION

The present invention relates to a process for reducing the concentration of nitrogen oxides in the effluent gases from manufacturing plants.

Waste nitrogen oxides are released into the atmosphere both in the tail gases from plants manufacturing nitric acid and also in the residual gases produced by various plants using nitric acid as a reagent. There has been a constant search to minimize these waste products both for reasons of economy and to avoid pollution by toxic materials. Renewed emphasis to this search has been given by anti-pollution legislation in many countries. Such legislation characteristically sets limits to the weight of nitrogen oxides which may be released per ton of production, and generally excludes resorting to the simple palliative of diluting the exhaust gas with air.

Conventional plants manufacturing nitric acid from ammonia produce exhaust gases which contain from 1,000 to 2,000 cm³ of nitrogen oxides per cubic meter of effluent. In contrast to this, recent trends in legislation seek to impose a maximum permissible exhaust of nitrogen oxides equal to 1.5 Kg. per ton of nitric acid produced, corresponding in conventional nitric acid plants to about 200 cm³ of nitrogen oxides per cubic meter of the non-diluted tail gases.

The presence of nitrogen oxides in the effluent gases from nitric acid production can be understood in light of the well known reactions whereby the catalytic combustion of ammonia gives rise to nitric oxide, NO, which is inert to water. In the presence of oxygen, NO then forms nitrogen dioxide, which either by itself or in association with NO as dinitrogen trioxide then reacts with water on absorption columns according to the following reactions (1) and (2) to form nitric acid but also NO:

    3N.sub.2 O.sub.3 + H.sub.2 O ⃡ 2HNO.sub.3 + 4NO (1)

    6NO.sub.2 or 3N.sub.2 O.sub.4 + 2H.sub.2 O ⃡ 4HNO.sub.3 + 2NO (2)

unless specifically stated otherwise, in the present Specification and claims N₂ O₃ shall be understood to designate the equilibrium mixture of NO + NO₂ ⃡ N₂ O₃ resulting from the partial decomposition of one mol of N₂ O₃. Likewise N₂ O₄ shall be understood to designate the equilibrium mixture N₂ O₄ ⃡ 2NO₂ resulting from the partial decomposition of one mol of N₂ O₄ ; or conversely NO₂ represents said equilibrium mixture resulting from the partial dimerization of one mol of NO₂.

The regeneration of NO in these self-oxidationreduction reactions of N₂ O₃ and N₂ O₄ respectively makes the overall conversion to nitric acid a progressive process. The operation is made more difficult by the fact that decreasing the concentration of either NO or oxygen retards the rate of forming the desired NO₂. As a result, it is extremely difficult to absorb the last traces of nitrogen oxides in the absorption system conventionally used in manufacturing nitric acid. Typically, the first two thirds of the absorption columns accomplish 98.5 percent of the operation but the remaining third is not sufficient to absorb completely the residual 1.5 percent and the tail gases contain at best 3 to 4 times as high a concentration of nitrogen oxides as is the present goal.

Attempts to lower the amounts of released nitrogen oxides by mere extension of the absorption system are fraught with difficult technical problems; also the additional installions would entail considerably increased investments.

In chemical plants utilizing nitric acid as a raw material, nitrogen oxide contaminants can be formed by the reaction of nitric acid with reducing agents. The products of such chemical reduction are sometimes limited to NO₂ (and N₂ O₄) but the reduction may proceed further to NO. The effluents of these plants therefore have compositions similar to those found in nitric acid manufacture and can be subjected to the same treatments.

Among the prior methods proposed to limit the quantity of nitrogen oxide gases released into the atmosphere, there can be mentioned alkaline absorption which has the advantage of not generating more nitric oxide NO but also the disadvantage of not capturing any free NO. The reactions taking place in alkaline absorption are:

    N.sub.2 O.sub.3 + Na.sub.2 O ⃡ 2NaNO.sub.2     ( 3)

    N.sub.2 O.sub.4 + Na.sub.2 O ⃡ NaNO.sub.2 + NaNO.sub.3 ( 4)

the Na₂ O, which can be introduced as NaOH or Na₂ CO₃ takes care of only NO₂ or its N₂ O₃ compound with NO and permits any excess NO to escape, being inert to the alkali but capable of later reacting with atmospheric oxygen to form NO₂.

The NO present in the gas to be treated usually is so dilute that its oxidation is very slow and the molar conversion of NO into NO₂ is generally much less than the 50 percent required for complete absorption. Furthermore, the solution of nitrites and nitrates produced by this procedure cannot be released into natural waters but they are too dilute for recovery as commercially usable salts to be practical.

It has also been proposed to destroy the excess nitrogen oxides by burning them, as for example along with natural gas as a fuel. This has been found to be economically unfeasible, not because of the relatively small, say 0.2 percent, loss in nitric acid yield, but because of the considerable investments and running expenses which are involved in operating installations to dispose of the gases produced in the burner. Other methods using catalytic combustion require cumbersome use of expensive gases like hydrogen and methane as well as large investments for costly catalysts.

SUMMARY OF THE INVENTION

A simple means has now been found whereby the concentration of nitrogen oxides contained in effluent gases can be substantially reduced in a manner easy to operate and not requiring large investments or operating costs.

Briefly stated, this invention comprises two steps whereby (1) the effluent gases are oxidized to convert the nitrogen oxides substantially to a mixture of NO₂ and N₂ O₃ and (2) the thus oxidized gases are passed through an aqueous solution containing an amount of hydrogen peroxide maintained at a level just sufficient to oxidize the NO₂ and N₂ O₃ to HNO₃.

DETAILED DESCRIPTION

It is known that H₂ O₂ does not oxidize nitric oxide NO to any appreciable extent when pure nitric oxide gas or nitric oxide diluted by nitrogen, argon, carbon dioxide or carbon monoxide is passed through an aqueous solution of H₂ O₂. However if the NO is first oxidized by a suitable agent such as oxygen, ozone or nitric acid, to a mixture of N₂ O₃ and N₂ O₄, these gases are easily absorbed in H₂ O₂ solution according to the overall reactions:

    N.sub.2 O.sub.3 + 2H.sub.2 O.sub.2 → 2HNO.sub.3 + H.sub.2 O (5)

    N.sub.2 O.sub.4 + H.sub.2 O.sub.2 → 2HNO.sub.3      (6)

hydrogen peroxide is commonly considered to be very stable in nitric acid solutions as long as the HNO₃ concentration is well under the concentration range which is conducive to the formation of unstable nitrous and nitric peroxide. The upper limit of thus permissible HNO₃ concentration is generally around 50 percent by weight but it depends also on temperature and hydrogen peroxide concentration. However, the present inventors have found that in the course of absorbing N₂ O₄ and N₂ O₃ conditions are developed which cause a high rate of decomposition of H₂ O₂ into H₂ O and O₂. When H₂ O₂ is introduced into an absorption system without the special precautions of this invention, the loss by decomposition is so great that up to three or four times the stoichiometric amount must be used to obtain a satisfactory decrease in the nitrogen oxide content of the effluent gases.

Present inventors have unexpectedly found that such loss of H₂ O₂ by decomposition is substantially eliminated if the H₂ O₂ is added gradually at a rate corresponding to the rate at which it reacts to oxidize the nitrogen oxide gases according to equations (5) and (6), substantially avoiding any excess of H₂ O₂.

This manner of operating has been found valuable in particular when NO is allowed to be oxidized by the oxygen present in the nitrogen oxide gases according to the reaction:

    NO + 1/2 O.sub.2 ⃡ NO.sub.2                    (7)

which makes the absorption on the kinetics of this NO₂ formation from NO and requires the introduction of H₂ O₂ at several points corresponding to the slowly progressive nature of the oxidation. When, on the other hand, the NO in the gas being absorbed has previously been oxidized either entirely or to the extent of 50 mol percent, injection of H₂ O₂ at a single point suffices. Such previous oxidation can be carried out with any suitable reagent, exemplarily nitric acid or ozone. The action of nitric acid is the reverse of equations (1) and (2), as follows:

    4NO + 2HNO.sub.3 ⃡ 3N.sub.2 O.sub.3 + H.sub.2 O (1')

    2NO + 4HNO.sub.3 ⃡ 3N.sub.2 O.sub.4 + 2H.sub.2 O (2')

these reactions are very rapid in the gas phase. They have been the subject of many research studies and their physical chemistry is well understood.

The process for reducing the nitrogen oxide content of effluent gases according to the present invention is characterized by the stepwise accomplishment of (a) an oxidation of NO to N₂ O₃ or NO₂ in the gas phase, followed by (b) an absorption in aqueous medium in presence of hydrogen peroxide carried out in such a manner that the quantity of H₂ O₂ introduced is just sufficient to oxidize N₂ O₃ or NO₂ to HNO₃ with substantial avoidance of excess H₂ O₂.

According to one embodiment the oxidation step is carried out by HNO₃ but any oxidation agent capable of oxidizing NO can be utilized therefor.

This process can be used to treat any residual gas containing nitrogen oxides and is of particular interest as a manner of reducing the concentration of nitrogen oxides in the effluents from nitric acid manufacturing plants, as well as for the treatment of residual gases in chemical plants using nitric acid, such as those for passifying stainless steel and for nitration of organic compounds.

When nitric acid is used to effect the first oxidation step, this can be carried out by mixing the effluent gases with gaseous nitric acid in any suitable manner at a pressure from 1 to 20 bars, a temperature from -10° C to +40° C.

The reaction can be brought about, for example, by bubbling the effluent gas through an aqueous solution of nitric acid or by spraying the acid into the gas. It has been established that it is most suitable to use an installation wherein perfect liquid-gas equilibrium is maintained.

The minimum feasible concentration of nitric acid is dependent on the composition of the treated gas and the temperature. It is generally desired to use nitric acid substantially free of nitrogen oxides in order to lessen the consumption of hydrogen peroxide.

When treating residual gases from plants manufacturing HNO₃, the concentration of nitrogen oxides in the treated gas is generally between about 1,000 and 2,000 cm³ /m³ which necessitates using HNO₃ of concentration 20-70% by weight at ambient temperature. In the case of other residual gases, HNO₃ concentration can vary from 20-85 percent.

In order to lower the consumption of H₂ O₂, it is possible to modify this first stage by removing from the main circuit aqueous nitric acid containing dissolved therein part of the NO₂ formed. The NO₂ can then be separated from the nitric acid and reintroduced upstream into the main circuit, while the nitric acid can be recycled.

In the second stage of the process, the gases are treated in liquid medium by passage through an aqueous solution receiving continuously H₂ O₂ in quantity such that the quantity of H₂ O₂ introduced is substantially that which is necessary to oxidize the N₂ O₄ and N₂ O₃ while avoiding an excess of H₂ O₂.

The treatment is generally effectuated at ambient temperature under a pressure of 1 to 20 bars on the gases which exit from the nitric acid oxidation at a temperature of -10° C to + 40° C. The reaction is very rapid; therefore any equipment can be used which accomplishes substantially complete contact between gas and H₂ O₂ solution. Inventors have established in particular that an installation corresponding to one theoretical plate serves particularly well.

The process for reducing the nitrogen oxide content of gaseous effluents according to the instant invention is particularly applicable to reducing the nitrogen oxide content of effluents from plants manufacturing nitric acid having absorption columns in stages. It suffices to subject one or several of such stages to the oxidation of the effluents by nitric acid and one or several other stages to the H₂ O₂ treatments.

In building new installations for manufacturing HNO₃, the use of this process permits constructing at substantially reduced overall cost a system which can alternatively be operated to yield tail gases having nitrogen oxide content typical of the prior art, or to obtain a high degree of purification when necessary, or to realize any compromise desired between these two extremes.

A particular advantage of the instant method is its flexibility in that the quantity of H₂ O₂ used can be adjusted to correspond to any desired degree of purification and to correspond to the changing variations during the course of production. The method is particularly adaptable to automatic control of the H₂ O₂ used in proportion to the changing concentration of nitrogen oxides in the residual gases.

In comparison to other known procedures for the treatment of residual gases containing nitrogen oxides utilizing H₂ O₂, it is unexpectedly economical since the consumption of H₂ O₂ is a unique function of the concentration of nitrogen oxides in the gas to be treated, and the oxidizing power of the H₂ O₂ is substantially completely utilized.

The following examples illustrate, without being limiting, the procedure according to this invention. In order to permit a better analysis of the phase being treated by H₂ O₂ and in view of the fact that the oxidation of nitrogen oxides by HNO₃ is well known, the examples relate to the fixation of different pure nitrogen oxides by H₂ O₂.

EXAMPLE 1

In the following procedures, a laboratory-scale fritted glass bubbler is used containing 50 cc of aqueous phase and connected with an absorption vessel containing standardized sodium hydroxide solution. In each example, there is passed through this system, at a rate of 150 liters per hour, a supply of nitrogen containing 2,000 cc of the specified nitrogen oxide per cubic meter. After equilibration of the liquid and gas phases, the operation is carried out for one hour and the following results are noted:

(1.1) The aqueous phase at the start is plain water. The nitrogen oxide is pure NO₂. In the absence of the H₂ O₂ used in the instant invention, there is formed only 253 mg. of HNO₃, corresponding to 49% of the acid stoichiometrically equivalent to 2,000 cc/m³ of NO₂ according to equation (2) above. The residual nitrogen oxides are totally absorbed in the sodium hydroxide, consisting therefore of a mixture of NO₂ and N₂ O₃ and the salts formed are a mixture of NaNO₂ and NaNO₃ very rich in nitrite.

(1.2) The aqueous phase is an aqueous solution of 40 grams of H₂ O₂ per liter. The nitrogen oxide is pure NO₂. The amount of HNO₃ formed is 633 mg. and 0.42 grams of H₂ O₂ have disappeared. The final concentration of H₂ O₂ is 31.6 grams/liter. The 633 mg. of HNO₃ corresponds to 121% of the acid which would theoretically result from the reaction (2) upon the 2,000 cc/m³ of NO₂ . The residual nitrogen oxides are totally absorbed in the sodium hydroxide, forming a 50/50 molar ratio of NaNO₃ and NaNO₂, corresponding to the effluent nitrogen oxide gas being entirely NO₂.

The consumption of H₂ O₂ is 1.23 mols of H₂ O₂ per mol of HNO₃ formed.

(1.3) The aqueous phase is an aqueous solution of 40 grams of H₂ O₂ per liter. The nitrogen oxide is pure NO.

The amount of HNO₃ formed is 24.4 mg. and 0.05 grams of H₂ O₂ have disappeared, the final concentration being 39 grams/liter. The 24.4 mg. of HNO₃ corresponds to 3% conversion of the NO supplied at 2,000 cc/m³ . The residual nitrogen oxides are not absorbed in the sodium hydroxide, only traces of NaNO₂ being formed. The consumption of H₂ O₂ is 3.8 mols of H₂ O₂ per mol of HNO₃ formed.

(1.4) The aqueous phase is an aqueous solution of 40 grams/liter of H₂ O₂ and the nitrogen oxide is a mixture of equal parts of NO and NO₂ corresponding to N₂ O₃.

The amount of HNO₃ formed is 470 mg. and 0.56 grams of H₂ O₂ have disappeared, the final concentration being 28.8 grams/liter.

The 470 mg. of HNO₃ corresponds to 180% of the acid which would be expected to form from 2,000 cc of N₂ O₃ per cubic meter, according to the reaction (1) above. The nitrogen oxide residuals are totally absorbed in the sodium hydroxide and form NaNO₂ with a small amount of NaNO₃.

The consumption of H₂ O₂ is 2.20 mols per mol of HNO₃ formed.

(1.5) The aqueous phase at the start is plain water, and the nitrogen oxide is pure NO₂. During the course of the hour's operation, there is continuously added by means of a measuring pump 0.07 cc/min. of an aqueous solution of 35 grams H₂ O₂ per liter.

The amount of HNO₃ formed is 460 mg. and the total disappearance of introduced H₂ O₂ is 147. The 460 mg. of HNO₃ corresponds to 87.5% of the stoichiometric amount corresponding to the 2,000 cc/m³ of NO₂. Without H₂ O₂ the corresponding yield in Example (1.1) is only 49 percent. The nitrogen oxide residuals are totally absorbed in the sodium hydroxide and form a 50/50 molar mixture of NaNO₃ and NaNO₂. The consumption of H₂ O₂ is only 0.59 mols per mol of HNO₃ formed as compared to 0.50 mols calculated theoretically from the reaction

    2 NO.sub.2 + H.sub.2 O.sub.2 → 2HNO.sub.3.

EXAMPLE 2

During a winter period when the exterior temperature does not exceed 10° C, the last absorption column of an installation for the nitric acid production of type 165/T/J having 16 plates operating under a pressure of 4 bars absolute, receives per hour 22000 cubic meter of gas containing 0.15 volume percent of nitrogen oxides and releases a residual gas containing 0.07% volume percent of nitrogen oxides.

There is introduced at the head of this column a stream of water at the rate of 7000 liters/hour and a stream of 35% by weight aqueous hydrogen peroxide (corresponding to a solution of 56 grams H₂ O₂ per liter) at a rate of 115 liters/hour, the usual feed supplied to the column at this location being 800 liters of water per hour.

After stabilization of the acid concentrations and gas compositions on the column, it is found that the residual gases contain between 0.017 and 0.018 percent of nitrogen oxides and this is confirmed by analysis of samples of the 10 highest plates.

By determination of the quantity of supplementary HNO₃ formed and of the consumption of H₂ O₂, it is observed that 4.40 mols of H₂ O₂ are consumed per mol of supplementary nitric acid formed. This calculation is a formal one since the acid which would normally form on these plates in the absence of H₂ O₂ also consumes H₂ O₂.

The use of H₂ O₂ under these conditions shows that it is possible to reduce the residual nitrous oxide concentration to 200 cc per cubic meter by consuming 6.9 Kg. of 100% H₂ O₂ per ton of nitric acid leaving the installation.

Example 3

During a summer period when the exterior temperature can reach 25° C the last column of the same installation as in Example 2 receives 0.35 volume percent of nitrogen oxides and releases at the rate of 22000 cm/h a residual gas containing 0.15 volume percent.

There is introduced at the last, sixteenth, plate and also at plate 11, two streams, each being respectively 30 liters per hour of 35% aqueous H₂ O₂, plate 16 receiving in addition 750 liters per hour of water for the absorption.

After establishment of a steady state on the column, it is observed that the residual nitrogen oxides have shifted toward 0.08 volume percent and that the acid produced has been increased by about 3.5 grams/liter at plate 9.

Plates 10 and 15 again have a considerable concentration of H₂ O₂, namely 2 and 6 grams/liter respectively, showing that the utilization of H₂ O₂ has been incomplete.

By analysis both of the supplementary quantity of HNO₃ formed and of the amount of H₂ O₂ consumed, it is observed that 1.6 mols of H₂ O₂ are consumed per mol of supplementary HNO₃ formed, which corresponds to 3.5 Kg. of H₂ O₂ per ton of HNO₃.

EXAMPLE 4

This example describes an industrial-scale operation in a column carrying out successively the oxidation of nitric oxide, NO, by nitric acid in the vapor phase, followed by the absorption of the resulting oxidized gases by hydrogen peroxide in aqueous medium.

The column used in this example contains 16 plates and is the last column of a unit producing 165 tons of nitric acid per day and operating under an absolute pressure of 4 bars. During a summer period when the exterior temperature can reach 25°C, this column receives gases containing 0.35% of nitrogen oxides. Without the modification of this example, the gas released at the rate of 22000 cubic meters per hour from this column contains 0.15% of nitrogen oxides.

The circuit of liquid flow through this column is changed so as to isolate two successive plates situated at midheight. The higher of these plates without liquid circulation serves as a "scrubber", the plate beneath it is supplied with 2 cubic meters per hour of an independent nitric acid stream whose concentration is maintained at about 52% HNO₃ by continuous withdrawal of a weak fraction (about 10%) from production. The volume of circulating acid is thus kept constant by a continuous removal to the storage tank.

Into each of the three plates immediately above those containing the concentrated acid there is introduced a 30 liters/hr. stream of 35% aqueous solution of hydrogen peroxide.

The plate situated at the top of the column (plate 16) receives, as in the preceding examples, the remaining water, flowing at about 750 liters/hr., which circulates through three plates before reaching those which receive hydrogen peroxide.

These modifications have the effect of diminishing very significantly the concentration of nitrogen oxides at the exit of the column. After the plates reach a steady state, the concentration of nitrogen oxides in the residual gases has settled to a level of 0.045% by volume.

The consumption of hydrogen peroxide corresponds to 5.1 Kg. of 100% H₂ O₂ per ton of nitric acid produced. 

What we claim is:
 1. A process for reducing the level of nitrogen oxide contaminants in an industrial effluent gas, which comprises (a) oxidizing the lower valenced nitrogen oxides of said gas to a mixture of substantailly N₂ O₃ and NO₂ and (b) treating the nitrogen oxide contaminants with an aqueous solution of hydrogen peroxide maintained at a concentration level that is substantially the stoichiometric quantity required to oxidize the N₂ O₃ and NO₂ to HNO₃.
 2. The process of claim 1 wherein the lower valenced nitrogen oxides are oxidized with nitric acid.
 3. The process of claim 2 wherein the nitric acid is introduced in the vapor state into the current of effluent gas containing said contaminants.
 4. A process according to claim 2 wherein the nitric acid is treated for the removal of dissolved nitrogen dioxide and the nitrogen dioxide is removed therefrom.
 5. A process according to claim 4 in which the removed nitrogen dioxide is reintroduced into the effluent gas stream at a point between the nitric acid treatment and the hydrogen peroxide treatment.
 6. A process according to claim 4 wherein the nitric acid is recycled back into nitric acid system after the dissolved nitrogen dioxide has been removed. 